The distribution and characteristics of internal flow field of the impeller with crack damage in service environment was investigated via numerical simulation with RNG k-ε turbulence model. The diffuser and volute are added based on the original impeller to simulate the internal flow field comprehensively. It was found that along the flow direction, the pressure, velocity, and temperature of the fluid increase continuously, and the maximum value appears near the outlet of the impeller. The maximum pressure and velocity in the crack area are distributed around the middle section and the trailing edge of the crack. Entropy production theory was applied in the study of internal flow, which reveals that the entropy production becomes larger around the crack. The further propagation of the crack is promoted by the opening force perpendicular to the entrance direction of the middle crack, the corrosion propagation at the rear edge of the crack, and the thermal deformation of the blade. The accelerated crack process will finally lead to the blade fracture accident.
The energy loss of the multi-stage centrifugal pump was investigated by numerical analysis using the entropy generation method with the RNG k-ε turbulence model. Entropy generation due to time-averaged motion and velocity fluctuation was mainly considered. It was found that the entropy generation of guide vanes and impellers account for 71.2% and 23.3% of the total entropy generation under the designed flow condition. The guide vanes are the main hydraulic loss domains and their entropy generation is about 9 W/K, followed by impellers. There are vortices at the tongue of the guide vane inlet as well as flow separations in the impellers, which lead to entropy generation. The fluid impacts the outer surface of the guide vanes, resulting in the increase in entropy generation. There are refluxes near the guide vane tongues which also increase the entropy generation of this part. The entropy generation distribution of the guide vanes and impellers was investigated, which found that the positive guide vane has more entropy generation compared with the reverse guide. The entropy generation of the blade suction surface is higher compared with the pressure surface. This study indicated that the entropy generation method has distinct advantages in the assessment of hydraulic loss.
Nuclear Thermal Propulsion (NTP) for deep space exploration is promising. Hydrogen is employed as the propellant to enhance the maximized specific impulse of NTP. Whereas, the low boiling point and density of hydrogen lead to several issues related to safety and storage. From the perspective of reactor physics, thermal hydraulics, material compatibility, and experience in the aerospace field, ammonia (NH3) is the alternative propellant to replace hydrogen. To analyze the specific impulse and system parameters of ammonia NTP, the hydrogen expander cycle NTP system analysis program PANES (Program for Analyzing Nuclear Engine Systems) is improved. The thermophysical properties library of ammonia has been increased in PANES. Based on the same thrust with hydrogen NTP, the design and analysis of ammonia NTP systems are carried out. This paper improves the PANES program and calculates the NTP performance parameters for potential alternative propellant ammonia, which provides a reference for the design of ammonia NTP system.
In this paper, a seven-stage high pressure centrifugal pump for water injection was taken as the research object. The steady state full flow field was numerically simulated by unstructured grid and Renormalization Group (RNG) k- ε turbulence model, and the results were verified by experiment. The flow characteristics of each flow passage component in the centrifugal pump under rated flow condition were analyzed, and the influence of wear-ring clearance flow on the full flow field was studied. The entropy production theory was applied to compare the distribution law of full flow field with and without wear-ring clearance at rated flow. The range and magnitude of the main energy loss in the pump were quantitatively analyzed. The results show that the guide vane area has the highest entropy production, which is the main energy loss part of the centrifugal pump. The existence of wear-ring clearance increases the entropy production of full flow field and has an adverse effect. The impeller wear-ring clearance has a greater influence on the full flow field than the guide vane wear-ring clearance.
The nuclear thermal propulsion (NTP) system can shorten the travel time in deep space exploration and reduce the initial mass of the launch vehicle due to its superior characteristics including high specific impulse and large thrust. Particle bed reactor (PBR) is one of the most appropriate reactor concepts to equip the NTP systems. To make the best use of PBR, the thermal-hydraulic design of the fuel element should be carefully considered and a flow-power matching technology should be developed. In this paper, a novel design employing a divergent hot gas channel is proposed to achieve a uniform flow distribution with lower maximum temperature and pressure drop. Through the analysis of the 1D modified momentum equation in the inlet plenum and hot gas channel, the model of pressure drop is established. Then, the differential equation of the ideal cross-section of the hot gas channel is derived. At last, the flow and heat transfer process in the fuel element with divergent hot gas channel is simulated by using computational fluid dynamics (CFD) code, and the reduction of pressure drop and temperature verifies the theoretical model. This study shows that the proposed design of the divergent hot gas channel can provide a new idea for thermal-hydraulic optimization of the PBR fuel element.
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